The reduction of nitric oxide emissions when operating diesel internal combustion engines may be a primary focus when trying to decrease emissions. One strategy in this case may be to recirculate exhaust gas from the exhaust gas tract into the internal combustion engine. For this purpose, a high pressure exhaust gas recirculating system (HP-EGR) may be used in which an exhaust gas recirculating line branches off from the exhaust gas tract directly downstream of the internal combustion engine and fluidly couples to the intake tract. Alternatively, or in combination with the high pressure exhaust gas recirculating system, a low pressure exhaust gas recirculating system (LP-EGR) may be used in which an exhaust gas recirculating line branches off from the exhaust gas tract downstream of a turbine and/or from one or multiple exhaust gas treatment systems. Low pressure exhaust gas recirculating systems have the advantage that the exhaust gas may be cleaned (e.g., filtered) and arrives cooler in the intake tract than in the case of a HP-EGR and therefore the intake air is cooler and consequently recirculates a larger quantity of LP-EGR. In comparison to using only a high pressure exhaust gas recirculating system, a low pressure exhaust gas recirculating system holds a greater potential for reducing the nitric oxide (NOx) emission of the internal combustion engine. The two exhaust gas recirculating systems are efficient in different areas of the engine rotation hold characteristic curve with the result that the use and throughput of the exhaust gas recirculating systems may be controlled in dependence upon prevailing parameters. The provision of the two systems also offers the advantage that, in the case of a degradation in one system, the other system may be used as a substitute.
Exhaust gas treatment devices may arranged in the exhaust gas tract so as to clean and/or filter the exhaust gas, the treatment devices may include one or more of nitric oxide absorption catalytic converters (lean NOx traps, LNT), particulate filters, and catalytic converters for selective catalytic reduction (SCR). The one or multiple SCR devices may be used to reduce nitric oxides present in the exhaust gas. Ammonia may be used as a reducing agent, said ammonia being introduced into the exhaust gas tract upstream of an SCR device in the form of an aqueous urea solution, for example AdBlue®, which may commercially available.
SCR devices may be arranged upstream and downstream of the branch of a low pressure exhaust gas line. If an SCR device is arranged upstream, ammonia that escapes from the SCR may be conveyed into the LP-EGR line and successively into the internal combustion engine. This may be undesirable since the combustion of ammonia may produce additional quantities of nitric oxides. Consequently, additional reducing agents are then desired that in turn may escape from the SCR as a result of slip.
A first SCR device that is arranged upstream of the branch of the low pressure exhaust gas recirculating line is located nearer to the internal combustion engine and is therefore exposed to higher exhaust gas temperatures than a second SCR device that is arranged further downstream. The first SCR may therefore exposed to a more intense thermal aging process. The ammonia absorption capacity of the first SCR may decrease as the SCR ages; which may contribute to the further increase in the quantity of recirculated ammonia.
The greater the quantity of ammonia being recirculated via the low pressure exhaust gas recirculating line to the internal combustion engine and combusted therein, the greater the quantity of reducing agent introduced into the exhaust gas tract in order to reduce the increased quantities of nitric oxide. In this case one inefficient point may be reached in which the use of the high pressure exhaust gas recirculating line may appear more efficient in comparison with the low pressure exhaust gas recirculating line. Thus, a strategy for the exhaust gas recirculation during the operation of the internal combustion engine compensating for ammonia is desired.
A first aspect of the disclosure relates to a method for controlling recirculated exhaust gas in an arrangement of an internal combustion engine having an exhaust gas tract, wherein at least one first SCR and also at least one first device for introducing ammonia into the exhaust gas tract are arranged upstream of the first SCR, and also at least one high pressure exhaust gas recirculating line branches off upstream of the first SCR and at least one low pressure exhaust gas recirculating line branches off downstream of the first SCR, the arrangement furthermore comprising a control device, and wherein exhaust gas is guided in a controlled via an algorithm through the low pressure exhaust gas recirculating line and/or high pressure exhaust gas recirculating line in dependence upon the quantity of ammonia desired and the magnitude of the nitric oxide emission, said method comprising operating the internal combustion engine when recirculating exhaust gas through the low pressure exhaust gas recirculating line or the high pressure exhaust gas recirculating line, determining a difference value for the quantity of ammonia desired by the first SCR between using the high pressure exhaust gas recirculating line (HP mode) and using the low pressure exhaust gas recirculating line (LP mode), determining a difference value for the magnitude of the nitric oxide emission between an HP mode and an LP mode, making a decision with regard to switching between HP mode and LP mode in dependence upon the determined difference values with regard to the quantity of ammonia desired and the magnitude of the nitric oxide emission, wherein the mode is selected that is characterized by virtue of a lower quantity of ammonia desired and a lower magnitude of nitric oxide emission.
The method in accordance with the disclosure may include finding the optimal exhaust gas recirculating strategy in which the magnitude of the nitric oxide emission to the environment may be kept as low as possible whilst consuming as little as possible of the reducing agent that is introduced. The method is therefore cost-effective and beneficial to the environment.
The difference values are determined in this case between the integrals of the values that are determined for high pressure exhaust gas recirculating operation and low pressure exhaust gas recirculating operation with regard to the quantity of ammonia desired or the magnitude of the nitric oxide emission. In this case, the values for the high pressure exhaust gas recirculating operation and low pressure exhaust gas recirculating operation respectively are calculated both under the condition that the low pressure exhaust gas recirculating system (low pressure mode) is currently being used as well as under the condition that the high pressure exhaust gas recirculating system (high pressure mode) is currently being used. At least one or multiple nitric oxide sensors may be arranged in the exhaust gas tract.
In one example, the issues described above may be addressed by a method comprising determining a first difference between a quantity of ammonia desired by an SCR device during a LP-EGR mode and a HP-EGR mode, determining a second difference between an amount of nitric oxide emission during the LP-EGR mode and the HP-EGR mode, and selecting one of the LP-EGR mode and the HP-EGR mode based on a comparison of the first and second differences. In this way, efficiency may be increased and reductant consumption may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.